Fuel injectors which meter fuel under high pressure depending on the driving condition of the vehicle and inject the fuel in a fine spray directly into the combustion chamber of the internal combustion engine are used for direct injection in internal combustion engines designed as gasoline engines. Such a fuel injector is described in German Patent No. DE 199 46 602 A1, for example. Such a fuel injector is inserted into a bore of a cylinder head of the internal combustion engine that seals a combustion chamber in such a way that the injector tip provided with injection openings protrudes into the combustion chamber and the sealing ring situated on the fuel injector creates a combustion chamber seal between the fuel injector and the bore wall of the cylinder head bore. The sealing ring typically having a rectangular cross section is made of a less elastic plastic, e.g., polytetrafluoroethylene (PTFE). The sealing ring is inserted into a ring groove present in the fuel injector prior to installation of the fuel injector in the cylinder head. Since the inner diameter of the seal adapted to the diameter of the groove base of the ring groove is smaller than the diameter of the ring groove on the groove edge, an enlarging tool 11—as schematically shown in FIG. 1—is placed on the tip of the fuel injector (FIG. 1a) and sealing ring 12 is pushed via enlarging tool 11 onto the fuel injector, whereby sealing ring 12 is enlarged and may thus be displaced on fuel injector 10 into ring groove 13 (FIG. 1b). Enlarging tool 11 is then removed from fuel injector 10 (FIG. 1c). Due to the low elasticity of the material of sealing ring 12, sealing ring 12 initially remains enlarged and is not fully pressed into ring groove 13 (FIG. 1c). Therefore, sealing ring 12 is reshaped with the aid of a so-called calibration tool 14 to its final outer diameter, its predefined sealing dimension. Calibration tool 14 is placed on the tip of fuel injector 10 and is pushed over sealing ring 12 for this purpose (FIG. 1d). To reshape sealing ring 12, calibration tool 14 has a hollow cone 15 which tapers against the slide-on direction with respect to the inner diameter, the smallest inner diameter on the side of hollow cone 15 facing away from fuel injector 10 being adapted to the desired sealing dimension of sealing ring 12. During slide-on of calibration tool 14, hollow cone 15 increasingly deforms sealing ring 12 until the desired outer diameter, the so-called sealing dimension, is achieved and simultaneously presses sealing ring 12 into ring groove 13. During this calibration of sealing ring 12, it is naturally pressed against the flank of ring groove 13 facing away from the combustion chamber. To prevent very high press-in forces and shearing off of sealing ring 12 during the installation of fuel injector 10 in the cylinder head, ring groove 13 filled by sealing ring 12 must not be overfilled by sealing ring 12. Sealing ring 12 is therefore dimensioned in such a way that a residual ring gap 16 remains in ring groove 12 after reshaping. Since, as already mentioned, sealing ring 12 is pressed during the calibration process onto the flank of ring groove 13 facing away from the combustion chamber, this residual ring gap 16 is formed at the groove flank facing the combustion chamber. This residual ring gap 16 is filled by combustion residues during operation of the internal combustion engine, thus additionally protecting sealing ring 12 from the high thermal load of the exhaust gases in the combustion chamber.
When using fuels containing ethanol and preferably in the case of central installation of the fuel injector in the cylinder head, these combustion residues are lacking and hot particles become caught in the residual air gap. The hot particles may burn through the sealing ring, resulting in a leaky combustion chamber seal and consequent failure of the fuel injector.